Abstract
Type IV pili are important for microcolony formation, biofilm formation, twitching motility, and attachment. We and others have shown that type IV pili are important for protein secretion across the outer membrane, similar to type II secretion systems. This study explored the relationship between protein secretion and pilus formation in Vibrio cholerae. The toxin-coregulated pilus (TCP), a type IV pilus required for V. cholerae pathogenesis, is necessary for the secretion of the colonization factor TcpF (T. J. Kirn, N. Bose, and R. K. Taylor, Mol. Microbiol. 49:81-92, 2003). This phenomenon is not unique to V. cholerae; secreted virulence factors that are dependent on the presence of components of the type IV pilus biogenesis apparatus for secretion have been reported with Dichelobacter nodosus (R. M. Kennan, O. P. Dhungyel, R. J. Whittington, J. R. Egerton, and J. I. Rood, J. Bacteriol. 183:4451-4458, 2001) and Francisella tularensis (A. J. Hager et al., Mol. Microbiol. 62:227-237, 2006). Using site-directed mutagenesis, we demonstrated that the secretion of TcpF is dependent on the presence of selected amino acid R groups at position five. We were unable to find other secretion determinants, suggesting that Y5 is the major secretion determinant within TcpF. We also report that proteins secreted in a type IV pilus biogenesis apparatus-dependent manner have a YXS motif within the first 15 amino acids following the Sec cleavage site. The YXS motif is not present in proteins secreted by type II secretion systems, indicating that this is unique to type IV pilus-mediated secretion. Moreover, we show that TcpF interacts with the pilin TcpA, suggesting that these proteins are secreted by the type IV pilus biogenesis system. These data provide a starting point for understanding how type IV pili can mediate secretion of virulence factors important for bacterial pathogenesis.